Single or double outlet don't matter...my factory made variac has a single outlet, but I plug a 3 way splitter in sometimes. It is a wise idea to put a circuit breaker on the input so if you plug a short into it it don't fry.
A good way to calibrate it once you have a box and knob with pointer is to plug a 40- 60w bulb in with a volt meter connected to the output. Every 10V make a line and label the output voltage every 5v make a dot. Best calibration would be with the input voltage at 120v or 125v. If you know you get say 50v at a knob position with an input of 120v you can take that ratio and mathematically determine the output if the line input voltage is something else.
A while ago I read the president got rid of that stupid law that was phasing out incandescent bulbs...We should see higher wattage incandescent bulbs return to store shelves.
